For driving motors used in a variety of different fields, there is increasing demand for smaller sizes and lighter weights, as well as increased efficiency, in line with the goal of reducing installation space and lowering cost. Along with this demand there is a desire for techniques that allow further improvement in, for example, the magnetic properties of sintered magnets to be used in driving motors.
R-T-B based rare earth sintered magnets have been used in the past as sintered magnets with high magnetic properties. It has been attempted to improve the magnetic properties of R-T-B based sintered magnets using heavy rare earth metals such as Dy and Tb, which have large anisotropic magnetic fields HA. However, with the rising costs of rare earth metal materials in recent years, there has been a strong desire to reduce the amount of usage of expensive heavy rare earth elements. In light of this situation, it has been attempted to improve magnetic properties by micronizing the structures of R-T-B based sintered magnets.
Incidentally, R-T-B based sintered magnets are produced by powder metallurgy methods. In production methods by powder metallurgy, first the starting material is melted and cast, to obtain an alloy strip containing the R-T-B based alloy. Next, the alloy strip is ground to prepare alloy powder having particle diameters of between several μm and several tens of μm. The alloy powder is then molded and sintered to produce a sintered compact. Next, the obtained sintered compact is worked to the prescribed dimensions. In order to improve the corrosion resistance, the sintered compact may be subjected to plating treatment if necessary to form a plating layer. It is thus possible to obtain an R-T-B based sintered magnet.
In the production method described above, melting and casting of the starting material are usually accomplished by a strip casting method. A strip casting method is a method in which the molten alloy is cooled with a cooling roll to form an alloy strip. In order to improve the magnetic properties of R-T-B based sintered magnets, it has been attempted to control the alloy structure by adjusting the cooling rate in the aforementioned strip casting method. For example, PTL 1 proposes obtaining an alloy strip comprising chill crystals, particulate crystals and columnar crystals with prescribed particle diameters, by a strip casting method.
FIG. 11 is a schematic cross-sectional view showing the cross-sectional structure of an R-T-B based alloy strip produced by a conventional strip casting method, along the thickness direction. The R-T-B based alloy strip contains columnar or resin-like crystals 2 of a R2T14B phase, as the main phase, and a grain boundary phase 4 such as an R-rich phase at the grain boundaries of the crystals 2.